Turn crazy in shock (4)
I believe the answer is:
stun
'shock' is the definition.
(both can mean to surprise)
'turn crazy' is the wordplay.
'turn' is a reversal indicator.
'crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'nuts' in reverse letter order is 'STUN'.
'in' acts as a link.
(Other definitions for stun that I've seen before include "Shock, flabbergast" , "'Shock, daze (4)'" , "deliver a knock-out blow" , "Confuse" , "Put out for the count" .)